Radar transponders have been utilized for many applications including missile, aircraft, and ship tracking and navigation for many years. They are generally used to receive, delay, amplify, and retransmit a radar signal to present a strong reference signal to the interrogating radar. However, most were non-coherent, answering back at a fixed frequency different from the radar transmitter. Some coherent units have been developed for missile and satellite tracking applications where knowing the velocity vector is critical to determining flight safety. However, these units are single frequency and have very narrow bandwidths of 10 MHz or less, and are tuned to the radar transmitter frequency which was typically never changed.
Frequency chirp or frequency hopping radar systems require a frequency agile transponder that could keep up with the radars to provide a steady tracking signal. A coherent transponder with approximately 1 GHz bandwidth was developed by the Navy in the early 1990s by expanding the earlier narrow bandwidth surface acoustic wave (SAW) and bulk acoustic wave (BAW) delay line technology with similar wider bandwidth components. Although this Navy unit is useful for applications where the radar frequency band can be confined to that of the transponder, there are applications where the frequency range of the interrogating radar exceeds that transponder's bandwidth.
The bandwidth of older designs is typically the bandwidth of SAW or BAW delay lines. It is desired to have an ultra-broad band transponder that has a bandwidth that covers the entire S, X and Ku microwave bands.